Carbon fiber for composite material

ABSTRACT

Oxidizing treatment of carbon fibers under specific conditions provides carbon fibers having oxygen concentration (O 1S  /C 1S  atom number ratio) in the surface part of the carbon fibers of 0.05 to 0.30 as measured by X-ray photoelectron spectroscopy, coefficients of water-extractable fraction of up to 2.0, and coefficients of tow spread of at least 1×10 -3 . These carbon fibers are excellent in adhesion to various matrix resins and when mixed as reinforcements with matrix resins, provide composite materials superior especially in impact resistance. The thus obtained composite materials are useful for aircraft, automobiles, and general industrial materials.

TECHNICAL FIELD

The present invention relates to carbon fibers effective for producingcarbon-fiber composite materials which are excellent in mechanicalproperties, particularly in compressive strength after impact. Compositematerials from the present carbon fibers can be used extensively foraircraft in the first place, and automobiles and general industrialpurposes.

BACKGROUND ART

Conventional carbon fibers for use in carbon-fiber composite materials,when a polyacrylonitrile-based precursor is used as starting fibers, arecommonly produced by subjecting first the precursor to flame-resistingtreatment in an oxidizing atmosphere at temperatures of 200° to 300° C.,then carbonizing the treated fibers in an inert atmosphere, andgenerally subjecting the carbonized fibers to gas-phase or liquid-phaseoxidation to enhance the adhesion of the carbon fibers to the matrix,followed by treating the fibers with a suitable sizing agent for thepurpose of inhibiting the fiber breaking and fuzzing which may takeplace during handling of the fiber. However, the composite materialscontaining these carbon fibers are still insufficient in the adhesion ofthe fibers to the matrix. In particular, the compressive strength afterimpact (CAI) is generally on a level of 193.2×10³ KPa (19.7 kg/mm²) whenthe impact force is 68.1 kg/cm (1500 lb in/in), as shown in Eur. PatentApplication Laid-Open No. 132853, Table IV, Example 35. Thus, it is thepresent situation that the CAI is extremely difficult to improve byusing a matrix having such a high heat resistance as shown in Example 35mentioned above.

While an average CAI of 45.3 ksi (31.8 kg/mm²) is obtained in Eur.Patent Application Laid-Open No. 133280, Examples 6, 7, and 8, whichaimed at improving the CAI, this type of composite has a tough layercalled an interlief between prepreg layers and this inserted layerhinders the raise of the fiber content by volume, gives two differentfaces to the prepreg, and makes the handleness inferior.

On the other hand, it is required in the aircraft industry to improvethe CAI to 27 kg/mm² or more in order to reduce the weight of planebodies and it is desired to develop a composite material containing nospecial layer for the purpose of satisfying this requirement.

DISCLOSURE OF INVENTION

In view of such problems as noted above, the present inventors madeintensive studies. As a result, it has been found that the compressivestrength after impact, in spite of using the same kind of matrix resin,can be improved markedly by using carbon fibers subjected to oxidizingtreatment which satisfy such specific conditions as stated below, andthe present invention has been accomplished.

The substance of the invention is that the compressive strength ofcarbon-fiber reinforced composites after impact, in spite of using thesame kind of matrix resin, can be improved markedly by using carbonfibers having the oxygen concentration (O_(1S) /C_(1S) atom numberratio) in the surface part of carbon fiber is from 0.05 to 0.30 asmeasured by X-ray photoelectron spectroscopy, a coefficient ofwater-extractable fraction as is defined later is up to 2.0, preferablyup to 0.5, a coefficient of tow spread as is defined later is at least1×10⁻³ mm/denier, and the amount of a sizing agent on the fiber is up to0.1% by weight.

BRIEF DESCRIPTION OF DRAWINGS

The drawings referred to in the description of the invention areexplained below.

FIG. 1 illustrates the method applied to measure the coefficient of towspread. The arrow indicates the direction of tow movement.

1: weight; 2, 3, and 4: 50-mmφ Rolls;

5: carbon fibers in tow form

FIGS. 2 and 3 are electron microscopic photographs (each magnificationfactor: 900) showing cross sections of test pieces preaared in Example 1and Comparative Example 1, respectively.

In each of FIGS. 2 and 3, a white part on the lower left side indicatesa length of 10 μ.

BEST MODE FOR CARRYING OUT THE INVENTION

The term "coefficient of water-extractable fraction" used herein means a200-nm light aboorbance determined in the following way: 1 to 5 g ofcarbon fibers are placed in a beaker of 8 to 16 cm inner diametercontaining distilled water is added in an amount of 11 times the weightof the carbon fibers. Then this beaker is placed in an ultrasoniccleaning vessel (oscillation frequency 43 kHz, high-frequency output 90W) having internal dimensions of 298 mm (L)×155 mm (W)×152 mm (H) andcontaining water heated up to 50°±5° C., and water extractable fractionof the carbon fibers are extracted thereby for 10 minutes. The fibersare removed and the extracting water is recovered and placed in a UVcell made of quartz having a cell length of 1 cm, and fresh distilledwater is placed in a reference liquid cell. The cells are scanned withUV rays of 187 to 400 nm wavelengths by using a UV spectrophotometer,and the 200-nm light absorbance of the extracting water is determined.

The coefficient of tow spread herein is a value determined by passing anuntwisted tow, as shown in FIG. 1, over a 50-mmφ bar (hard chromiumplated and #200 satin finished) at an access angle of 30° and a leavingangle of 45° and at a linear velocity of 1 m/min under a tension of 75mg/denier, and measuring the width (mm) of the tow spread on the bar.The value is expressed by the width divided by the whole denier of thetow. The distance between bars 1 and 2 is 30 cm and the distance betweenbars 2 and 3 is 50 cm.

The object carbon fibers may be obtained from eitherpolyacrylonitrile-based precursors or pitch type precursors and havetensile modulus of elasticity of at least 19 ton/mm², tensile strengthsof at least 250 kg/mm², and elongations of at least 1.5%.

The carbon fibers of the invention need to have the oxygen concentration(O_(1S) /C_(1S) atom number ratio) of 0.05 to 0.30, determined by X-rayphotoelectronic spectroscopy, at the surface. When this amount is lessthan 0.05, the adhesion of the carbon fibers to the matrix resin isinsufficient and when the amount exceeds 0.30, the fiber strengthlowers; hence such amounts are undesirable. For producing carbon fibershaving oxygen-containing functional groups in an amount of 0.05 to 0.30;an example of suitable methods in the case of liquid-phase treatment isto apply a positive voltage to carbon fibers in an aqueous nitric acidsolution through the mediation of a metallic guide roll arrangedimmediately before a treatment vessel, thereby providing an electriccharge of 60 to 600 coulombs per 1 g of carbon fibers between the fiberand a platinum cathode plate arranged in the treating solution; and anexample of suitable methods in the case of gas-phase treatment is totreat carbon fibers in an air atmosphere containing 1 to 5% by volumeozone, at a temperature of 100° to 200° C. for a period of 1 to 5minutes.

These surface-treated carbon fibers are immediately treated in generalwith a sizing agent. In this case, any sizing agent may be used providedthat the relation of the sizing agent and the amount thereof issatisfied so that the coefficient of tow spread may become at least1×10⁻³ mm/denier. In any case, preferred sizing agents have highcompatibility with the matrix. Less coefficients of tow spread than1×10⁻³ are undesirable since such a tow will be split insufficientlyinto individual discrete filaments and the adhesion of carbon fibers tothe matrix resin will be impaired.

For satisfying the above-mentioned requirement, the amount of sizingagent adhering is up to 0.1%, preferably up to 0.01%, by weight.

The amount of sizing agent adhering is measured by the washing methodwith sulfuric acid in accordance with Japanese Industrial Standard R7601 6.8.2, Testing Methods for Carbon Fibers.

When the amount of sizing agent adhering exceeds 0.1% by weight, thespread ability of the tow becomes worse and the filaments stick togetherlocally and hence the penetration of the matrix resin into the stickingportions tends to be inhibited. However, the adhesion of larger amountsof sizing agent than 0.1% by weight is allowed if the tow is once splitinto individual discrete filaments by blowing it with hot air, passingthe tow over a bar, beating the tow under tension, or combining thesemethods, whereby the coefficient of tow spread can be increased to alevel of 1×10⁻³ mm/denier. In view of affinity for the matrix resin, thesizing agent to be used herein is preferably bisphenol A diglycidylether, the reaction product thereof with bisphenol A, or such a compoundrepresented by the formula ##STR1## which is produced by reacting theglycidyl group of bisphenol A diglydyl ether with a polyether alcoholderived from ethylene oxide.

When the coefficient of water-extractable fraction in the invention islarger than 2.0, layers having relatively low bonding strength arepresent on surfaces of carbon fibers and the matrix resin is bonded tocarbon fibers through these layers. Hence, it is considered that abonding between carbon fibers and the matrix resin can be destroyedunder relatively low load.

Therefore it is desirable to reduce these layers, the level of whichneeds to be such low as to give a coefficient of water-extractablefraction of up to 2.0. For reducing the coefficient of water-extractablefraction of carbon fibers, an effective method comprises subjectingcarbon fibers produced to a surface oxidizing treatment, followed bywashing them with, for example, water or aqueous solution of pH 4-12. Itis effective to use ultrasonic cleaning, heating, or induction heating,during the washing.

EXAMPLES

The following examples illustrate the present invention.

The compressive strength after impact was determined in the followingway:

In accordance with NASA Reference Publication 1092 (Standard tests forToughened Resin Composites, Revised Edition), a panel having dimensionsof 4"×6"×0.25" is fixed over a steel table having an opening of 3"×5", aweight of 4.5 kg having a nose of 1/2"R is dropped on the center of thepanel to give an impact of 1500 lb in per 1 in of panel thickness, andthen the panel is subjected to a compression test to determine thecompressive strength after impact.

EXAMPLES 1-4 AND COMPARATIVE EXAMPLES 1-4

Acrylic tows each consisting of 6000 filaments of 1.5 deniers infilament size were prepared from a polymer of 0.20 in specific viscosity[ηsp] constituted of 98 wt. % of acrylonitrile, 1 wt. % of methylacrylate, and 1 wt. % of methacrylic acid, by wet spinning usingdimethylformamide as a solvent, followed by stretching the spun fibersover a hot water bath at a draw ratio of 5:1, washing with water,drying, and dry stretching at 170° C. and at a draw ratio of 1.3:1.

The degree of orientation, n, of these fibers was found to be 90.3% byX-ray diffraction.

These acrylic fibers were treated for flame-resisting by passing themfor 60 minutes through a hot-air circulation type of flame-resistingfurnace having a temperature profile of three stages: 220° C.-240°C.-260° C., while the fibers were 15% stretched by difference inrevolution speed between rolls so far as the fiber density reached 1.22g/cm³ and then the local shrinkage of the fibers was inhibited byequalizing the revolution speeds of rolls with which the fibers werebrought into contact. Thus the flame-resisting treatment was completed.

In the next place, the flame-resisting fibers were passed for 3 minutesthrough a stream of pure N₂ gas in a first carbonization furnace at 600°C. while the fibers were 10% stretched. The fibers were furtherheat-treated under a tension of 400 mg/denier in the same atmosphere ina second carbonization furnace in which the maximum temperature was1200° C., thereby yielding carbon fibers (I-a) having physicalproperties of 503 Kg/mm² in tensile strength, 24 ton/mm² in tensilemodulus of elasticity, 1,790 g/cm³ in density, and 0.4 g/m in mass perunit length.

Carbon fibers obtained above were then left staying in tow form for 3minutes in an air containing 1.8 vol % of ozone at 200° C. Then, 100-mportions of the tows were wound up separately around perforatedstainless steel bobbins of 70 mm in outer diameter and immersed inboiling water at a bath ratio of 1/20 for varying periods of time, thuspreparing carbon fibers (I-b) of groups exhibiting severallycoefficients of water-extractable fractions of 2.5, 0.8, 0.7, and 0.5.The oxygen concentration (O_(1S) /C_(1S) atom number ratio) in thesurface part of these carbon fibers were from 0.15 to 0.20. Then, thesegroups of carbon fibers were passed through a solution of Epikote 834 (abisphenol A type of epoxy resin made by Yuka-Shell Co., Ltd.) in methylethyl ketone to prepare carbon fibers (I-c) of groups to which thesizing agent adhered severally in amounts of 0.01, 0.07, 0.2, and 0.5wt. %.

A matrix resin was prepared as was described in Example 1 of JapanesePatent Application Laid-Open No. 17289/85. Namely, 9 parts of4,4'-diaminodiphenyl sulfone was added to 100 parts of an epoxy resin(epoxy resin (A), i.e. Epikote 828, made by Shell Chemical Co.). Themixture was placed in a heating vessel equipped with a stirrer and waspolymerized with stirring at 150° C. for 4 hours. Then the mixture wasdischarged and spread in film form on a panel cooled with ice to stopthe polymerization. To 100 parts of the obtained precondensate (B) wasadded 3 parts of N-(3,4-dichlorophenyl)-N',N'-dimethylurea and themixture was stirred at 50° C. giving a pasty substance. 60 Parts of thispasty substance was mixed with 40 parts of methyl ethyl ketone to form auniform solution. Each of the above different sorts of carbon fibersshown in Table 1 was wound up around a drum while being impregnated withthis solution, and was dried and then cut open, thereby giving a prepregin which carbon fibers were arranged unidirectionally (fiber arealweight 145 g/m², resin content 33 wt. %). Portions of each prepreg werelaminated together in the quasi-isotropic state of [+45/0/-45/90]_(4S),and cured at 180° C. for 2 hours, forming a composite panel. Fibercontents by volume of the thus obtained composites were as shown inTable 1. Results of measuring then the compressive strengths afterimpact are also shown in Table 1. It is evident from these results thathigh compressive strengths after impact are provided by carbon fibershaving low coefficients of water-extractable fraction, high coefficientsof tow spread, and small amounts of sizing agent adhering.

                  TABLE 1                                                         ______________________________________                                                              Formed panel                                            Carbon fiber used       Com-                                                                             Amount pressive                                          Coefficient                                                                             Coefficient                                                                              of sizing                                                                            strength                                          of water- of tow     agent  after                                             extractable                                                                             spread     adhering                                                                             impact  Vf*                                 No.   fraction  (mm/denier)                                                                              (wt %) (kg/mm.sup.2)                                                                         (%)                                 ______________________________________                                        Exam-                                                                         ple                                                                           1     0.5       2.8 × 10.sup.-3                                                                    0.01   28      60                                  2     0.8       2.8 × 10.sup.-3                                                                    0.01   26      61                                  3     0.7       1.4 × 10.sup.-3                                                                    0.07   27      60                                  4     0.5       2.8 × 10.sup.-3                                                                    0.01   28.5    60                                  Com-                                                                          par.                                                                          Exam-                                                                         ple                                                                           1     2.5       2.8 × 10.sup.-3                                                                    0.01   24      61                                  2     0.5       0.8 × 10.sup.-3                                                                    0.2    24      62                                  3     2.5       0.8 × 10.sup.-3                                                                    0.2    22      60                                  4     0.5       0.4 × 10.sup.-3                                                                    0.5    23      61                                  ______________________________________                                         *Fiber content by volume                                                 

FIGS. 2 and 3 show electron microscopic photographs (each magnificationfactor: 900) of break surfaces of test specimens of Example 1 andComparative Example 1, respectively, after tests for compressivestrength after impact. It is apparent from these photographs that theadhesion between the fiber and the matrix varies greatly with thecoefficient of water-extractable fraction and carbon fibers having highcoefficients of water-extractable fraction exhibit weak adhesion to thematrix.

EXAMPLES 5-8 AND COMPARATIVE EXAMPLES 5-8

Results shown in Table 2 were obtained by following the procedure ofExample 1 but using (i) carbon fibers provided with the oxygenconcentration (O_(1S) /C_(1S) atom number ratio) in the surface part ofcarbon fiber is from 0.18 to 0.22 by flowing an electric charge of 200coulombs per 1 g of the fiber in 2% nitric acids to oxidize the carbonfibers anodically and then washing the carbon fibers with hot water and(ii) as a matrix resin the same composition as of Example 1 of JapanesePatent Application Laid-Open No. 215314/84.

                  TABLE 2                                                         ______________________________________                                                              Formed panel                                            Carbon fiber used       Com-                                                                             Amount pressive                                          Coefficient                                                                             Coefficient                                                                              of sizing                                                                            strength                                          of water- of tow     agent  after                                             extractable                                                                             spread     adhering                                                                             impact  Vf*                                 No.   fraction  (mm/denier)                                                                              (wt %) (kg/mm.sup.2)                                                                         (%)                                 ______________________________________                                        Exam-                                                                         ple                                                                           5     0.5       2.8 × 10.sup.-3                                                                    0.01   29      60                                  6     0.8       2.8 × 10.sup.-3                                                                    0.01   27.5    60                                  7     0.7       1.4 × 10.sup.-3                                                                    0.07   27      61                                  8     0.5       2.8 × 10.sup.-3                                                                    0.01   28      60                                  Com-                                                                          par.                                                                          Exam-                                                                         ple                                                                           5     2.5       2.8 × 10.sup.-3                                                                    0.01   23      60                                  6     0.5       0.8 × 10.sup.-3                                                                    0.2    22.5    62                                  7     2.5       0.8 × 10.sup.-3                                                                    0.2    22      62                                  8     0.5       0.4 × 10.sup.-3                                                                    0.5    23      60                                  ______________________________________                                         *Fiber content by volume                                                 

EXAMPLES 9-12 AND COMPARATIVE EXAMPLES 9-12

Results shown in Table 3 were obtained by following the procedure ofExample 1 but using the same composition as of Example 2 of Eur. PatentApplicatinn Laid-Open No. 133281.

                  TABLE 3                                                         ______________________________________                                                              Formed panel                                            Carbon fiber used       Com-                                                                             Amount pressive                                          Coefficient                                                                             Coefficient                                                                              of sizing                                                                            strength                                          of water- of tow     agent  after                                             extractable                                                                             spread     adhering                                                                             impact  Vf*                                 No.   fraction  (mm/denier)                                                                              (wt %) (kg/mm.sup.2)                                                                         (%)                                 ______________________________________                                        Exam-                                                                         ple                                                                            9    0.5       2.8 × 10.sup.-3                                                                    0.01   29      60                                  10    0.8       2.8 × 10.sup.-3                                                                    0.01   28      60                                  11    0.7       1.4 × 10.sup.-3                                                                    0.07   27.5    61                                  12    0.5       2.8 × 10.sup.-3                                                                    0.01   28      61                                  Com-                                                                          par.                                                                          Exam-                                                                         ple                                                                            9    2.5       2.8 × 10.sup.-3                                                                    0.01   22      60                                  10    0.5       0.8 × 10.sup.-3                                                                    0.2    23      60                                  11    2.5       0.8 × 10.sup.-3                                                                    0.2    22      61                                  12    0.5       0.4 × 10.sup.-3                                                                    0.5    21      62                                  ______________________________________                                         *Fiber content by volume                                                 

COMPARATIVE EXAMPLE 13

Portions of carbon fibers (I-a) prepared in Example 1 were subjected intow form to oxidizing treatment by causing them to stay severally for0.5 and 8 minutes in a 200° C. air containing 1.8 vol % of ozone, andwere treated with boiling water in the same manner as in Example 1 togive a coefficient of water-extractable fraction of 0.5. In this case,the respective oxygen concentration (O_(1S) /C_(1S) atom number ratio)in the surface part of carbon fibers is 0.03 and 0.4. Then, 0.01 wt % ofthe same sizing agent as used in Example 1 was caused to adhere, andcomposites were prepared in the same manner as in Example 1. Therespective compressive strengths after impact were found to be 18 kg/mm²(Vf 61%) and 25 kg/mm² (Vf 60%). It is evident therefrom that sufficientperformance cannot be obtained when the amount of oxygen concentration(O_(1S) /C_(1S) atom number ratio) in the surface of carbon fibers istoo small. It also can be seen that the performance is lowered when thisamount is too large.

EXAMPLES 13-16 AND COMPARATIVE EXAMPLES 14-17

Carbon fibers (I-a) having physical properties of 458 kg/m² in tensilestrength, 30.2 ton/mm² in tensile modulus of elasticity, 1.770 g/cm² indensity, and 0.39 g/m in mass per unit length were prepared according tothe procedure of Example 1 except that the maximum atmospherictemperature of the carbonizing conditions in the second carbonizationfurnace was changed to 1800° C. These carbon fibers were anodicallyoxidized by flowing an electric charge of 250 coulombs per 1 g of thefiber in a 5% aqueous solution of ammonium hydrogencarbonate, and werewashed with hot water, providing carbon fibers having 0.2 to 0.21(O_(1S) /C_(1S)) of oxygen concentration (O_(1S) /C_(1S) atom numberratio) in the surface part of the carbon fibers. Results shown in Table4 were obtained therefrom by following the procedure of Example 1 butusing the same composition as of Example 2 of Japanese PatentApplication Laid-Open No 58424/85.

                  TABLE 4                                                         ______________________________________                                                              Formed panel                                            Carbon fiber used       Com-                                                                             Amount pressive                                          Coefficient                                                                             Coefficient                                                                              of sizing                                                                            strength                                          of water- of tow     agent  after                                             extractable                                                                             spread     adhering                                                                             impact  Vf*                                 No.   fraction  (mm/denier)                                                                              (wt %) (kg/mm.sup.2)                                                                         (%)                                 ______________________________________                                        Exam-                                                                         ple                                                                           13    0.5       2.8 × 10.sup.-3                                                                    0.01   30      60                                  14    0.8       2.8 × 10.sup.-3                                                                    0.01   27.5    61                                  15    0.7       1.4 × 10.sup.-3                                                                    0.07   27      61                                  16    0.5       2.8 × 10.sup.-3                                                                    0.01   29      60                                  Com-                                                                          par.                                                                          Exam-                                                                         ple                                                                           14    2.5       2.8 × 10.sup.-3                                                                    0.01   21      61                                  15    0.5       0.8 × 10.sup.-3                                                                    0.2    22      60                                  16    2.5       0.8 × 10.sup.-3                                                                    0.2    20      60                                  17    0.5       0.4 × 10.sup.-3                                                                    0.5    21      61                                  ______________________________________                                         *Fiber content by volume                                                 

EXAMPLE 17 AND COMPARATIVE EXAMPLE 18

Composites were prepared from the same respective carbon fibers as usedin Example 1 and Comparative Example 1, by following the respectiveprocedures of Example 1 and Comparative Example 1 except that the carbonfibers were impregnated with a matrix resin shown in Example 35 of Eur.Patent Application Laid-Open No. 132853. The composites were measuredfor compressive strength after impact. Results of the measurement wereas shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                              Formed panel                                            Carbon fiber used       Com-                                                                             Amount pressive                                          Coefficient                                                                             Coefficient                                                                              of sizing                                                                            strength                                          of water- of tow     agent  after                                             extractable                                                                             spread     adhering                                                                             impact  Vf*                                 No.   fraction  (mm/denier)                                                                              (wt %) (kg/mm.sup.2)                                                                         (%)                                 ______________________________________                                        Exam-                                                                         ple                                                                           17    0.5       2.8 × 10.sup.-3                                                                    0.01   29      61                                  Com-                                                                          par.                                                                          Exam-                                                                         ple                                                                           18    2.5       2.8 × 10.sup.-3                                                                    0.01   21      59                                  ______________________________________                                         *Fiber content by volume.                                                

INDUSTRIAL APPLICABILITY

The carbon fibers of the present invention, having excellent properties,that is to say, the amount of oxygen concentration (O_(1S) /C_(1S) atomnumber ratio) in the surface part of the carbon fibers is from 0.05 to0.3 as measured by X-ray photoelectron spectroscopy, the coefficient ofwater-extractable fraction is up to 2.0, and the coefficient of towspread is at least 1×10⁻³ mm/denier, are best suited as reinforcementsfor addition to various matrix resins. Because of being superiorespecially in compressive strength after impact, composite materialscomprising matrix resins reinformed with carbon fibers of the inventionare useful for aircraft, automobiles, and general industrial materials.

We claim:
 1. Carbon fibers which are obtained by flame-resistingtreating of acrylic fiber and then carbonizing at a temperature of 1000°C. or more, and thereafter treating the resulting carbon fiber with anoxidation treatment in liquid or gas phase in one step, for compositematerials excellent in impact resistance, characterized in that theoxygen concentration (O_(1S) /C_(1S) atom numer ratio) in the surfacepart of said carbon fibers is from 0.05 to 0.2 as measured by X-rayphotoelectron spectroscopy, the coefficient of water-extractablefraction is up to 2.0, and the coefficient of tow spread is at least1×10⁻³ mm/denier.
 2. The carbon fibers of claim 1, wherein the amount ofa sizing agent adhering onto the fiber is up to 0.1% by weight.
 3. Thecarbon fibers of claim 1, characterized by having tensile strengths of250 kg/mm², tensile modulus of elasticity of at least 19 ton/mm2, andelongations of at least 1.5%.